Internal and External Memories

Internal and External Memories

Fig. 14.5.1 shows the basic memory structure for 8051. It can access up to 64 K program memory and 64 K data memory. The 8051 has 4 K bytes of internal program memory and 256 bytes of internal data memory.

1. Internal RAM Organization

The 8051 has 128-byte internal RAM. It is accessed using RAM address register. The Fig. 14.5.2 shows the organization of internal RAM. As shown in the Fig. 14.5.2 internal RAM of 8051 is organized into three distinct areas :

• Register bank

• Bit addressable

• General purpose.

a. 8051 Register Banks (Working Registers)

The first 32-bytes from address OOH to 1FH of internal RAM constitute 32 working registers. They are organized into four banks of eight registers each. The four register banks are numbered 0 to 3 and are consists of eight registers named R₀ to R₇.

Each register can be addressed by name or by its RAM address.

Only one register bank is in use at a time. Bits RSO and RSI in the PSW determine which bank of registers is currently in use.

On reset, the bank 0 is selected and hence it is a default register bank. Register banks when not selected can be used as general purpose RAM.

 b. Bit / Byte Addressable

The 8051 provides 16 bytes of a bit-addressable area. It occupies RAM byte addresses from 20H to 2FH, forming a total of 128 (16 × 8) addressable bits.

An addressable bit may be specified by its bit address of 00H to 7FH, or 8 bits may form any byte address from 20H to 2FH.

For example, bit address 4EH refers bit 6 of the byte address 29H.

 c. General Purpose RAM

The RAM area above bit addressable area from 30H to 7FH is called general purpose RAM. It is addressable as byte.

2. ROM Space in the 8051

The 8051 has 4 kbyte of internal ROM with address space from 0000H to OFFFH. It is programmed by manufacturer when the chip is built. This part cannot be erased or altered after fabrication. This is used to store final version of the program. It is accessed using program address register.

Review Questions

1. What do you understand by bit addressable RAM in 8051 microcontroller ? AU : Dec.-10

2. Discuss the internal memory organization of the 8051 microcontroller. AU : Dec.-10, May-11, Marks 16

3. Discuss about the organization of internal RAM and special function registers of 8051 microcontroller in detail.

4. Discuss in datail internal data memory organization of microcontroller 8051. AU : Dec.-19, Marks 8

5. Explain the program memory and data memory structure of 8051 microcontroller. AU : Dec.-11, Marks 8

7. Explain the RAM structure of 8051 microcontroller.